In electrophotography, also known as Xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
Although excellent toner images may be obtained with multilayered belt or drum photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators, and printers are developed, there is a greater demand on print quality. The delicate balance in charging image and bias potentials, and characteristics of the toner and/or developer, are desirably be maintained. This places additional constraints on the quality of photoreceptor manufacturing, and thus on the manufacturing yield.
Imaging members are generally exposed to repetitive electrophotographic cycling, which subjects the exposed charged transport layer or alternative top layer thereof to mechanical abrasion, chemical attack and heat. This repetitive cycling leads to gradual deterioration in the mechanical and electrical characteristics of the exposed charge transport layer.
Providing a protective overcoat layer is a conventional means of extending the useful life of photoreceptors. Conventionally, for example, a polymeric overcoat has been utilized as a robust overcoat design for extending the lifespan of photoreceptors. The conventional polymeric overcoat includes (i) a polyol binder, (ii) a melamine-formaldehyde curing agent; (iii) a hole transport material; (iv) an acid catalyst; and (v) a leveling agent coated from an alcoholic solution. However, the conventional overcoat formulation generates unacceptably high levels of free formaldehyde remaining after curing, which poses a human health hazard. The free formaldehyde arises from the melamine cross-linking component of the overcoat formulation.
Despite the various approaches that have been taken for forming imaging members there remains a need for improved imaging member design, to provide improved imaging performance and longer lifetime, reduce human and environmental health risks, and the like.